This invention relates to sulfonated perfluorocyclobutane polymers suitable for use as ion-conducting membranes, including proton exchange membranes, and methods for making same.
Ion conducting membranes (ICM""s) are used in electrochemical cells as solid electrolytes. The ICM is the central portion of a membrane electrode assembly (MEA), typically sandwiched between two catalytic electrode layers. ICM""s allow ions to move between an anode and a cathode, thereby allowing electrical current to flow in an external circuit that connects the two electrodes.
In a typical hydrogen/oxygen fuel cell, the ions conducted by the ICM are protons (H+). To make an efficient fuel cell, the ICM must not conduct electrons and must have little or no permeability to the fuel gasses.
U.S. Pat. No. 5,620,807 discloses the use of a perfluorocyclobutylene polymer to support electrically conductive particles in a gas-permeable porous outer layer of a membrane electrode assembly. This reference does not teach the use of a perfluorocyclobutylene polymer as an ion conducting membrane.
Briefly, the present invention provides a sulfonated aromatic perfluorocyclobutane polymer meeting a condition selected from: a) the equivalent weight of the polymer is 5000 or less; and b) the proton conductivity of the polymer at 25xc2x0 C. is 0.01 Siemens per centimeter (S/cm) or higher. In addition, the present invention provides a sulfonated aromatic perfluorocyclobutane polymer comprising units according to the formula: 
wherein xe2x80x94Bxe2x80x94 is 1,2-perfluorocyclobutylene (C4F6); each xe2x80x94Xxe2x80x94 is independently selected from the group consisting of: a bond, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94NR1xe2x80x94, and xe2x80x94R2xe2x80x94, wherein R1 is C1-C25 substituted or unsubstituted, saturated or unsaturated alkyl or aryl and R2 is C1-C25 substituted or unsubstituted, saturated or unsaturated alkylene or arylene; xe2x80x94Arxe2x80x94 is a substituted or unsubstituted C1-C30 at least divalent aromatic hydrocarbon group; and n is a non-negative integer chosen independently for each of the repeating units such that the average for the polymer is greater than 0.
In another aspect, the present invention provides a process for preparing a polymer comprising the step of sulfonating a perfluorocyclobutane polymer to form a sulfonated perfluorocyclobutane polymer, wherein said sulfonated perfluorocyclobutane polymer is sufficiently sulfonated as to meet a condition selected from: a) the equivalent weight of the polymer is 5000 or less; and b) the proton conductivity of the polymer at 25xc2x0 C. is 0.01 S/cm or higher. The step of sulfonation can be accomplished, e.g., by use of sulfuric acid, or by applying a halosulfonic acid to add sulfonyl halide groups and thereafter hydrolyzing the sulfonyl halide groups to form sulfonic acid groups.
The present invention also provides a process for preparing a sulfonated perfluorocyclobutane polymer comprising: A) providing a perfluorocyclobutane polymer including sulfur-containing groups; and B) converting said sulfur-containing group into a sulfonic acid group to form a sulfonated perfluorocyclobutane polymer, wherein said sulfonated perfluorocyclobutane polymer is sufficiently sulfonated as to meet a condition selected from: a) the equivalent weight of the polymer is 5000 or less; and b) the proton conductivity of the polymer at 25xc2x0 C. is 0.01 S/cm or higher.
In this document:
xe2x80x9cC(number)xe2x80x9d refers to a chemical moiety containing the indicated number of carbon atoms;
xe2x80x9csubstitutedxe2x80x9d when used without reference to a particular substituent, means substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.; and
xe2x80x9csulfonationxe2x80x9d refers to addition of a sulfonate group, a group that is readily hydrolyzed to a sulfonate such as a sulfonyl halide, or a sulfur-containing proton-conducting group such as a sulfonimide.
The polymers of the present invention can achieve a combination of high ionic conductivity, low electron conductivity, good dimensional stability, and good mechanical strength, making them excellent choices for use in ion conducting membranes. These polymers are useful as proton exchange membranes in fuel cell applications.